StorageManager: Improve exception handling

When "getPrimaryStorageSize" provides a path
to "readLong", the option that the path
doesn't exist is expected, since it tries
all paths from "INTERNAL_STORAGE_SIZE_PATHS"
until there is success.

This patch makes us catch the "FileNotFoundException"
and "NumberFormatException" seperately.

For the above reason a "FileNotFoundException"
is now treated as an information only.
The "NumberFormatException" and other exceptions
are now treated as error since those are not
expected to happen.

Change-Id: I5316f9c3108e36c31b27dc5df8bf8ac4d4257629
Signed-off-by: Alex Naidis <alex.naidis@linux.com>